AbstractsEarth & Environmental Science

The geology of the dolomitic magnesite deposits of Grenville township is described in detail and an hypothesis oonoerning the origin of the orebodies is presented. The deposits were formed by injection of an aqueous differentiate from the Morin magma, into fractured sillimanaite-garnetnear the contact with cuartzite. Following the crystallization of magnesite from the injected bodies, aqueous residues attacked the country rocksand formed the zone of hydrothermal alteration now referred to as the ore zone. The solutions acting during the period of hydrothermalalteration were richer in lime than the original differentiate. Consequently, dolomite formed from them as veins and replacements. In places, thelater solutions attacked the earlier formed magnesite and altered it to dolomite.The literature on magnesite deposits of the world is reviewed and the various types of magnesite deposits are described. A comparison shows thatthe Grenville deposits are more closely related to the sagvandite group of igneous rocks with primary magnesite, than to other deposits suchas those of the Veitsch type which are replacements of limestone.